A “flush” in mushroom cultivation refers to a distinct, synchronized wave of mushroom growth that occurs after the mycelium has fully colonized its substrate. The first flush typically provides the largest yield, but the substrate, or “cake,” retains significant potential for subsequent harvests if properly managed. This guide details the steps necessary to rehydrate and revitalize the substrate to maximize its potential for a second and possibly third flush.
Preparing the Substrate for Rehydration
The immediate steps following the initial harvest are fundamental for ensuring a successful subsequent flush and preventing contamination. It is necessary to remove all residual mushroom matter from the surface of the substrate block, including small pins, aborted fruits, and any remaining stem bases left behind after picking. These organic remnants can quickly become vectors for mold or bacterial growth if they are allowed to decompose on the substrate’s surface.
When harvesting, the preferred technique is often a gentle twist-and-pull motion, which aims to remove the entire mushroom cleanly from the substrate surface. If cutting is used, the slice should be made flush with the substrate to avoid leaving behind stumps that can rot. Thoroughly cleaning the surface maximizes the area available for pinning in the next cycle and prepares the substrate for optimal water absorption during the rehydration process. Allowing the cleaned block to rest for 24 to 48 hours in a clean, low-humidity area can also help the mycelium recover before the soak.
The Rehydration Process
Mature mushrooms are approximately 90% water by weight, and the first harvest significantly depletes the moisture content of the substrate. The most common and effective method for replenishing this lost moisture is “dunking” the substrate block. This technique fully submerges the block in clean water, allowing the mycelium to draw in the water it needs to trigger the next fruiting cycle.
To begin, place the substrate block into a clean container and fill it with non-chlorinated water at room temperature, ideally between 65 and 70 degrees Fahrenheit. The block will naturally float due to its porous structure, so it is important to weigh it down to ensure complete submersion. A clean, water-filled bag or a heavy object placed on top of a piece of aluminum foil can be used to keep the block under the water line.
The duration of the soak can vary depending on the substrate’s size and density, but a soak time between 4 and 24 hours is typically recommended. For most standard substrate blocks, a soak of 6 to 12 hours is sufficient. Exceeding 12 hours is generally discouraged as it increases the risk of bacterial growth and can starve the mycelium of oxygen.
After the soak, the block must be allowed to drain thoroughly for 30 to 60 minutes before being returned to the fruiting chamber. Removing all excess surface water is important, as standing water on the substrate can also encourage the growth of contaminants.
Re-establishing Fruiting Conditions
Following the rehydration process, the substrate is ready to be placed back into the fruiting environment, where the conditions that stimulate pinning must be re-established. The primary environmental triggers for a new flush are a fresh air exchange (FAE), high humidity, and a proper temperature range. The soaked block needs a constant supply of fresh air to lower the carbon dioxide levels that accumulated during the initial colonization phase.
Increasing the FAE helps to stimulate the formation of new mushroom pins, often within three to seven days after rehydration. Simultaneously, the humidity within the fruiting chamber must be maintained at a high level, typically between 85% and 95%, to prevent the new pins from drying out. Maintaining the appropriate temperature range, usually slightly cooler than the colonization temperature, signals the mycelium to shift its energy toward reproduction. This stable, slightly adjusted environment provides the necessary cues for the mycelium to begin its next wave of growth.
Managing Subsequent Flushes
The substrate block, having expelled a significant amount of its stored energy and nutrients in the first flush, will exhibit diminishing returns in subsequent harvests. While a second flush is highly probable, and a third is often achievable, the yield will typically decrease with each successive round. The first flush often accounts for 60% to 80% of the total potential yield, with later flushes producing fewer, though sometimes larger, mushrooms.
Most healthy substrate blocks can produce between two and four productive flushes before the mycelium is fully exhausted. It is important to monitor the block closely for signs that its productive life is ending, such as a significant lack of pinning after rehydration or the development of contamination. Signs like the appearance of green or other colored molds, or a foul odor, indicate that the substrate is failing and should be discarded to prevent the spread of contaminants to other cultivation projects.